Nuclear effects on the transverse momentum spectra of charged particles in pPb collisions at root s(NN)=5.02 TeV

DOI 10.1140/epjc/s10052-015-3435-4

Regular Article - Experimental Physics

Nuclear effects on the transverse momentum spectra of charged

particles in pPb collisions at √s

= 5.02 TeV

CMS Collaboration∗ CERN, Geneva, Switzerland

Received: 18 February 2015 / Accepted: 1 May 2015 / Published online: 29 May 2015

© CERN for the benefit of the CMS collaboration 2015. This article is published with open access at Springerlink.com

Abstract Transverse momentum spectra of charged parti-cles are measured by the CMS experiment at the CERN LHC in pPb collisions at√sNN = 5.02 TeV, in the range 0.4 <

pT < 120 GeV/c and pseudorapidity |ηcm| < 1.8 in the proton–nucleon center-of-mass frame. For pT < 10 GeV/c, the charged-particle production is asymmetric aboutηcm = 0, with smaller yield observed in the direction of the proton beam, qualitatively consistent with expectations from shad-owing in nuclear parton distribution functions (nPDF). A pp reference spectrum at√s = 5.02 TeV is obtained by

inter-polation from previous measurements at higher and lower center-of-mass energies. The pT distribution measured in pPb collisions shows an enhancement of charged particles with pT > 20 GeV/c compared to expectations from the pp reference. The enhancement is larger than predicted by per-turbative quantum chromodynamics calculations that include antishadowing modifications of nPDFs.

1 Introduction

The central goal of the heavy ion experimental program at ultra-relativistic energies is to create a system of deconfined quarks and gluons, known as a quark–gluon plasma (QGP), and to study its properties as it cools down and transitions into a hadron gas. A key tool in the studies of the QGP is the phenomenon of jet quenching [1], in which the partons produced in hard scatterings lose energy through gluon radi-ation and elastic scattering in the hot and dense partonic medium [2]. Since high transverse momentum quarks and gluons fragment into jets of hadrons, one of the observable consequences of parton energy loss is the suppression of the yield of high- pTparticles in comparison to their production in proton–proton (pp) collisions. This suppression, studied as a function of the pT and pseudorapidity (η) of the pro-duced particle, is usually quantified in terms of the nuclear modification factor, defined as

_{e-mail:cms-publication-committee-chair@cern.ch} RAB(pT, η) = 1 TAB d2NAB/d pTdη d2_σpp_{/d p} Tdη, (1)

where NABis the particle yield in a collision between nuclear species A and B,σpp is the corresponding cross section in pp collisions, andTAB is the average nuclear overlap func-tion [3] in the AB collision (in the case of proton–nucleus col-lisions, the quantityTAB = TpA is called average nuclear thickness function). If nuclear collisions behave as incoher-ent superpositions of nucleon–nucleon collisions, a ratio of unity is expected. Departures from unity are indicative of final-state effects such as parton energy loss, and/or initial-state effects such as modifications of the nuclear parton dis-tribution functions (nPDF) [4]. The nPDFs are constrained by measurements in lepton–nucleus deep-inelastic scattering (DIS) and Drell–Yan (DY) production of dilepton pairs from qq annihilation in proton–nucleus collisions [5]. In the small parton fractional momentum regime (x  0.01), the nPDFs are found to be suppressed relative to the proton PDFs, a phe-nomenon commonly referred to as “shadowing” [6]. At small

x, where the parton distributions are described theoretically

by non-linear evolution equations in x, gluon saturation is predicted by the color glass condensate models [7–9]. For the x regime 0.02  x  0.2, the nPDFs are enhanced (“antishadowing”) relative to the free-nucleon PDFs [5].

To gain access to the properties of the QGP produced in heavy ion collisions it is necessary to separate the effects directly related to the hot partonic medium from those that are not, referred to as “cold nuclear matter” effects. In par-ticular, nPDF effects are expected to play an important role in the interpretation of nuclear modification factors at the CERN LHC. Unfortunately, the existing nuclear DIS and DY measurements constrain only poorly the gluon distribu-tions over much of the kinematic range of interest. High- pT hadron production in proton–nucleus (or deuteron–nucleus) collisions provides a valuable reference for nucleus–nucleus collisions, as it probes initial-state nPDF modifications over a wide kinematic range and is expected to be largely free from the final-state effects that accompany QGP production [10].

The measurements of the nuclear modification factors of neutral pions and charged hadrons in the most central gold– gold (AuAu) collisions at the relativistic heavy ion collider (RHIC) [11–14] revealed a large suppression at high pT, reaching an RAAas low as 0.2. In contrast, no such suppres-sion was found at mid-rapidity in deuteron–gold collisuppres-sions at the same energy [15–18]. These findings established parton energy loss, rather than initial-state effects [19], as the mech-anism responsible for the modifications observed in AuAu collisions.

At the LHC, the charged-particle suppression in lead–lead (PbPb) collisions persists at least up to a pTof 100 GeV/c [20, 21]. In proton–lead (pPb) collisions, the ALICE Collabo-ration reported no significant deviations from unity in the charged-particle RpPbup to pT≈ 50 GeV/c [22]. The analysis presented here used data from CMS to extend the measure-ment of the charged-particle RpPb out to pT ≈ 120 GeV/c, with the aim of evaluating initial-state effects over a kine-matic range similar to that explored through measurements in PbPb collisions [20].

Proton–nucleus collisions have already been used to assess the impact of cold nuclear matter on jet production at the LHC. The transverse momentum balance, azimuthal angle correlations, and pseudorapidity distributions of dijets have been measured as a function of the event activity, and no significant indication of jet quenching was found [23]. When normalized to unity, the minimum-bias dijet pseudorapidity distributions are found to be consistent with next-to-leading-order (NLO) perturbative quantum chromodynamic (pQCD) calculations only if nPDF modifications are included [24]. Similarly, inclusive jet RpPbmeasurements are also found to be consistent with NLO pQCD predictions that include nPDF modifications [25,26]. The measurement of the charged-particle spectra presented in this paper provides a compari-son to theory that is sensitive to smaller x values than those accessible in the jet measurements. However, it should be noted that the charged-particle RpPbis dependent upon non-perturbative hadronization effects, some of which, such as gluon fragmentation into charged hadrons, are poorly con-strained at the LHC energies [27].

The pT distributions of inclusive charged particles in pPb collisions at a nucleon–nucleon center-of-mass energy of 5.02 TeV are presented in the range of 0.4 < pT < 120 GeV/c. The measurement is performed in several pseu-dorapidity intervals over |ηcm| < 1.8. Here ηcm is the pseudorapidity in the proton–nucleon center-of-mass frame. The nuclear modification factor is studied at mid-rapidity, |ηcm| < 1, and the forward-backward asymmetry of the yields, Yasym, defined as

Y_asym(a,b)(pT) = _−a −b dηcmd2Nch(pT)/dηcmd pT b a dηcmd2Nch(pT)/dηcmd pT , (2)

is presented for three pseudorapidity intervals, where a and

b are positive numbers, and Nchis the yield of charged par-ticles.

Due to their wide kinematic coverage, the measurements are expected to provide information about the nPDFs in both the shadowing and antishadowing regions. In particular, the effects of shadowing are expected to be more prominent at forward pseudorapidities (in the proton-going direction), where smaller x fractions in the nucleus are accessed.

In the absence of other competing effects, shadowing in the Pb nPDFs would result in values of Yasym > 1 at low pT(i.e., small x). The effects of antishadowing can be probed with the RpPb measurement at larger pT values of 30  pT  100 GeV/c that correspond approximately to

0.02  x  0.2. Antishadowing in the nPDFs may increase

the yield of charged particles in pPb collisions compared with expectations from the yield in pp collisions.

2 Data selection and analysis

2.1 Experimental setup

A detailed description of the CMS detector can be found in Ref. [28]. The CMS experiment uses a right-handed coordi-nate system, with the origin at the nominal interaction point (IP) at the center of the detector, and the z axis along the beam direction. The silicon tracker, located within the 3.8 T mag-netic field of the superconducting solenoid, is used to recon-struct charged-particle tracks. Consisting of 1440 silicon pixel detector modules and 15,148 silicon strip detector mod-ules, totaling about 10 million silicon strips and 60 million pixels, the silicon tracker measures the tracks of charged par-ticles within the pseudorapidity range|η| < 2.5. It provides an impact parameter resolution of≈ 15 μm and a pT reso-lution of about 1.5 % for particles with pTof 100 GeV/c. An electromagnetic calorimeter (ECAL) and a hadron calorime-ter (HCAL) are also located inside the solenoid. The ECAL consists of more than 75, 000 lead tungstate crystals, arranged in a quasi-projective geometry; the crystals are distributed in a barrel region (|η| < 1.48) and in two endcaps that extend out to |η| ≈ 3.0. The HCAL barrel and endcaps, hadron sampling calorimeters composed of brass and scintillator plates, have an acceptance of|η|  3.0. The hadron forward calorimeters (HF), consisting of iron with quartz fibers read out by photomultipliers, extend the calorimeter coverage out to|η| = 5.2, and are used in offline event selection. Beam Pick-up Timing for the eXperiments (BPTX) devices were used to trigger the detector readout. They are located around the beam pipe at a distance of 175 m from the IP on either side, and are designed to provide precise information on the LHC bunch structure and timing of the incoming beams. The

detailed Monte Carlo (MC) simulation of the CMS detector response is based on Geant4 [29].

This measurement is based on a data sample correspond-ing to an integrated luminosity of 35 nb−1, collected by the CMS experiment in pPb collisions during the 2013 LHC running period. The center-of-mass energy per nucleon pair was√sNN = 5.02 TeV, corresponding to per-nucleon beam energies of 4 TeV and 1.58 TeV for protons and lead nuclei, respectively. The data were taken with two beam configura-tions. Initially, the Pb nuclei traveled in the counterclockwise direction, while in the second data-taking period, the beam directions were reversed. Both data sets, the second one being larger by approximately 50 %, were analyzed independently, yielding compatible results. To combine data from the two beam configurations, results from the first data-taking period are reflected along the z-axis, so that in the combined anal-ysis, the proton travels in the positive z andη directions. In this convention, massless particles emitted atηcm = 0 in the nucleon–nucleon center-of-mass frame will be detected atηlab= 0.465 in the laboratory frame. A symmetric region aboutηcm = 0 is used in the data analysis, resulting in a selected pseudorapidity range of|ηcm| < 1.8.

2.2 Event selection

The CMS online event selection employs a hardware-based level-1 (L1) trigger and a software-based high-level trigger (HLT). A minimum-bias sample is selected first by the L1 requirement of a pPb bunch crossing at the IP (as measured by the BPTX), and an HLT requirement of at least one recon-structed track with pT > 0.4 GeV/c in the pixel tracker. For most of the 5.02 TeV data collection, the minimum-bias trig-ger is significantly prescaled because of the high instanta-neous LHC luminosity. To increase the pTreach of the mea-surement, a set of more selective triggers is also used: addi-tional L1 requirements are imposed to select events that have at least one reconstructed calorimeter jet with an uncorrected transverse energy of ET > 12 GeV, and ET > 16 GeV. These event selections are complemented by additional HLT requirements that select events based on the presence of at least one track with pT > 12 GeV/c (for L1 ET > 12 GeV), or with pT > 20 or 30 GeV/c (for L1 ET> 16 GeV) recon-structed in the pixel and strip tracker.

The above triggering strategy allows for the optimiza-tion of the data-taking rate while adequately sampling all pT regions, including collecting all events containing very

high-pTtracks. The track trigger with a pTthreshold of 12 GeV/c records about 140 times more events with high- pTtracks than the minimum-bias trigger, the track pT > 20 GeV/c trigger enhances this with an additional factor of about 8, while the track pT > 30 GeV/c trigger that is not prescaled, increases the number of events with a high- pT track by yet another factor of about 2.

In the offline analysis, additional requirements are applied. Events are accepted if they have (i) at least one HF calorime-ter tower on both the positive and negative sides of the HF with more than 3 GeV of total energy, (ii) at least one recon-structed collision vertex with two or more associated tracks, and (iii) a maximum distance of 15 cm along the beam axis between the vertex with the largest number of associated tracks and the nominal IP. Beam-related background is sup-pressed by rejecting events where less than 25 % of all recon-structed tracks are of good quality [30].

An event-by-event weight factor accounts for correcting the measured charged-particle spectra in pPb collisions to a detector-independent class of collisions termed as “double-sided” (DS) events, which are very similar to those that pass the offline selection described above. A DS event is defined as a collision producing at least one particle in the pseudo-rapidity range−5 < ηlab < −3 and another in the range 3 < ηlab < 5, each with proper lifetime τ > 10−18s and energy E > 3 GeV [31]. The performance of the minimum-bias and high- pTsingle-track triggers, as well as the offline criteria in selecting DS events, is evaluated with simulations using the hijing MC generator [32], version 1.383 [33], and the correction factors are computed as a function of the event multiplicity. An efficiency of 99 % is obtained for the minimum-bias trigger and a negligible correction (i.e., 100 % efficiency) for the high- pTtrack-triggered events. The correction factor is also evaluated using an epos [34] sim-ulation and, based on the difference between both genera-tors, a slightly pT-dependent systematic uncertainty of 1 % is assigned to the final spectra.

During the pPb data taking period, about 3 % of the recorded events contained more than one pPb collision. To reduce potential bias in the measurements arising from such “pileup”, events with multiple reconstructed vertices are removed if the longitudinal distance between the vertices along the beamline is greater than a specific value that is related to the uncertainty of the vertex position. This value is also dependent on the number of tracks associated with each vertex and ranges from 0.2 cm for vertex pairs with at least 25 tracks associated with each vertex, to 3 cm for vertex pairs with only 3 tracks associated with the vertex having the fewest associated tracks. Simulated hijing events are used to tune the pileup-rejection algorithm in order to reduce the number of erroneously eliminated single-collision events to a negligible fraction, and still maintain a high rejection ciency for genuine pileup events. The pileup-rejection effi-ciency is found to be 92± 2 %, which is confirmed by using a low bunch intensity control sample in data.

To obtain inclusive particle spectra up to pT≈ 120 GeV/c, data recorded with the minimum-bias and high- pT track triggers must be combined appropriately. The correspond-ing weight factors are computed by countcorrespond-ing the number of events that contain leading tracks (defined as the track with

[c/GeV] T dN/dp Leading ch 1/N -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 1 Minimum bias Track12 Track20 Track30 -1 = 5.02 TeV, L = 35 nb NN s CMS pPb |<2.4 lab η | [GeV/c] T

Leading charged particle p

5 10 15 20 25 30 35 40 45 Ratios 0 0.2 0.4 0.6 0.8 1 1.2 1.4

Fig. 1 Top Charged-particle yields for the different triggers

normal-ized to the number of leading charged particles with pT> 0.4 GeV/c in double-sided events, N_chLeading, as a function of leading-track pT. The track-triggered distributions are normalized by the number of leading tracks in regions not affected by the rapid rise of the trigger efficiency near threshold. Bottom Ratios of the leading-track pTdistributions for the four different triggers. The stars indicate the ratio of the 12 GeV/c over the minimum-bias samples, the circles the 20 over the 12 GeV/c samples, and the squares the ratio of the 30 over the 20 GeV/c track-triggered spectra

the highest pTin the event) in the range of|ηlab| < 2.4 with

pTvalues in regions not affected by trigger thresholds, i.e., where the trigger efficiency of the higher-threshold trigger is constant relative to that of the lower-threshold trigger. The ratio of the number of such events in the two triggered sets of data are used as weight factors. For example, the region above which the track trigger with a pTthreshold of 12 GeV/c has constant efficiency is determined by comparing the pT distri-bution of the leading tracks to that of the minimum-bias data. Similarly, the constant efficiency region of the 20 GeV/c track trigger is determined by comparison to the 12 GeV/c track trigger, and the 30 GeV/c trigger to the 20 GeV/c trigger. The regions of constant efficiency for each trigger, as a function of leading charged-particle pT, are shown in Fig.1. The 12, 20, and 30 GeV/c triggers have constant efficiencies above a lead-ing charged-particle pTof 14, 22, and 32 GeV/c, respectively. The weight factors are then computed using the leading-track

pTclasses of 14 < pT < 22 GeV/c, 22 < pT < 32 GeV/c, and pT> 32 GeV/c for the three high-pTtriggers. The com-bined uncertainty in these normalizations is dominated by the matching of the 12 GeV/c track-triggered events to the minimum-bias events.

Some events selected by the track triggers in Fig. 1are observed to result in a leading charged-particle pTbelow the corresponding trigger threshold. This can happen if theη of the track above threshold is outside theη range considered in the analysis, and because the final track reconstruction— described in Sect. 2.3—is more robust and selective than the fast-tracking algorithm implemented in the HLT. When the HLT selects an event based on a misreconstructed track, it is often the case that the track is not found in the final reconstruction. To determine the inclusive particle spectrum, events are first uniquely classified into leading-track pT classes in the pseudorapidity range in which the spectrum is being measured. The spectra are constructed by taking events from the minimum-bias, 12 GeV/c track, 20 GeV/c track, and 30 GeV/c track trigger, respectively, for each bin. A 4 % sys-tematic uncertainty on the possible trigger-bias effect is esti-mated from MC simulations. This procedure was verified in a data-driven way by constructing a charged-particle spectrum from an alternative combination of event samples triggered by reconstructed jets. Both final spectra, triggered by tracks and jets, are found to be consistent within the associated sys-tematic uncertainty.

2.3 Track reconstruction

The pTdistribution in this analysis corresponds to that of pri-mary charged particles, defined as all charged particles with a mean proper lifetime greater than 1 cm/c, including the products of strong and electromagnetic decays, but exclud-ing particles originatexclud-ing from secondary interactions in the detector material. Weak-decay products are considered pri-mary charged particles only if they are the daughters of a particle with a mean proper lifetime of less than 1 cm/c, pro-duced in the collision.

Charged particles are reconstructed using the standard CMS combinatorial track finder [35]. The proportion of mis-reconstructed tracks in the sample is reduced by applying an optimized set of standard tracking-quality selections, as described in Ref. [35]. A reconstructed track is considered as a primary charged-particle candidate if the statistical signif-icance of the observed distance of closest approach between the track and the reconstructed collision vertex is less than three standard deviations, under the hypothesis that the track originated from this vertex. In case an event has multiple reconstructed collision vertices but is not rejected by the pileup veto, the track distance is evaluated relative to the best reconstructed collision vertex, defined as the one associated with the largest number of tracks, or the one with the lowest

χ2_{if multiple vertices have the same number of associated} tracks. To remove tracks with poor momentum reconstruc-tion, the relative uncertainty of the momentum measurement

σ(pT)/pTis required to be less than 10 %. Only tracks that fall in the kinematic range of|ηlab| < 2.4 and pT> 0.4 GeV/c

are selected for analysis to ensure high tracking efficiency (70–90 %) and low misreconstruction rates (<2%).

The yields of charged particles in each pT andη bin are weighted by a factor that accounts for the geometrical accep-tance of the detector, the efficiency of the reconstruction algo-rithm, the fraction of tracks corresponding to a non-primary charged particle, the fraction of misreconstructed tracks that do not correspond to any charged particle, and the fraction of multiply-reconstructed tracks, which belong to the same charged particle.

The various correction terms are estimated using simu-lated minimum-bias pPb events from the hijing event gen-erator. To reduce the statistical uncertainty in the correction factors at high pT, samples of hijing events are also mixed with pp dijet events from the pythia MC generator [36] (ver-sion 6.423, tune D6T with CTEQ6L1 PDF for 2.76 TeV, tune Z2 for 7 TeV [37]).

The efficiency of the charged-particle reconstruction as well as the misreconstruction rates are also evaluated using pPb events simulated with epos. Differences between the two MC models are mostly dominated by the fraction of charged particles consisting of strange and multi-strange baryons that are too short-lived to be reconstructed unless they are produced at very high pT. Such differences in parti-cle species composition, which are largest for partiparti-cles with 3 pT  14 GeV/c, are propagated as a systematic uncer-tainty in the measured spectra. Below this pT range, the strange baryons are only a small fraction of the inclusive charged particles in either model, and the difference in recon-struction efficiency between particle species has less impact at even larger pT, as high- pTmulti-strange baryons can be directly tracked with high efficiency. Additional checks were performed by changing cutoffs imposed during track selec-tion and in the determinaselec-tion of the corresponding MC-based corrections. The corresponding variations in the corrected yields amount to 1.2–4.0 % depending on the pTregion under consideration, and are included in the systematic uncertainty. Finite bin-widths and finite transverse momentum resolu-tion can deform a steeply falling pTspectrum. The data are corrected for the finite bin-width effect as they will be com-pared to a pp reference spectrum obtained by interpolation. The binning corrections are derived by fitting the measured distribution and using the resulting fit function as a prob-ability distribution to generate entries in a histogram with the same pTbinning as used in the measurement. The cor-rection factors are then obtained from the ratio of entries in the bins of the histogram to the fit function evaluated at the centers of the bins. This correction amounts to 0–12 %, depending on pT. A similar method is used to evaluate the “smearing” effect of the finite pT resolution on the binned distributions. It is found that the momentum measurement, which has a resolution ofσ (pT)/pT ≈ 1.5 % near a pTof 100 GeV/c, is sufficiently precise to only have a negligible

effect on the measured spectra and therefore no correction factor is applied. To account for possible incorrect determi-nation of the momentum resolution from the simulation, the effects were again evaluated after increasing the value of

σ(pT)/pTby an additional 0.01, which produces a maximal distortion in the spectrum at a given pTof less than 1 %. 2.4 Proton–proton reference spectrum

The pPb collisions occur at a center-of-mass energy of 5.02 TeV per nucleon pair. At this collision energy, no proton– proton collisions have been provided by particle accelerators yet. The pp results closest in center-of-mass energy (√s)

and with similar acceptance are those measured at 2.76 and 7 TeV by the CMS experiment [20,38]. The determination of the nuclear modification factor RAB resides in an inter-polated reference spectrum to be constructed from data at higher and lower energies. We follow the direct interpolation method developed in Ref. [38] using measured pT spectra from inelastic collisions with|η| < 1.0 at√s = 0.63, 1.8,

and 1.96 TeV collision energies from CDF [39,40], and 0.9, 2.76, and 7 TeV collision energies from CMS [20,38]. This interpolation can be performed either as a function of pTor as a function of xT≡ 2pTc/√s.

Since the pT or xT values of the input data points are often different for each measurement performed at the vari-ous collision energies, each spectrum must first be fitted as a function of pTor xT. An interpolation is performed by fit-ting each of the spectra to a power-law dependence, and the resulting residuals to first- or third-order splines. The fitted spectra are then interpolated to determine the value of the reference spectrum at√s = 5.02 TeV using a second-order

polynomial in the plane of the log-log invariant production

vs.√s, as shown in Fig.2. For the pT-based direct interpo-lation, data from only two of the six spectra are available at

pT> 30 GeV/c, which implies that the pT-based direct inter-polation is well constrained only at low pT. On the other hand, the xT-based interpolation is well constrained at high pTfor √

s = 5.02 TeV, so it is natural to combine the reference

distributions from these two direct interpolation methods. The final pp reference spectrum is obtained by combining the and xT-based reference spectra as follows. The pT-based reference is chosen for pTbelow 12.5 GeV/c, and the

xT-based result above 13.5 GeV/c; between these two pT val-ues a linear weighting is implemented for the two references. The systematic uncertainty in the pp reference spectrum is determined through changing both the specific method of interpolation, as well as the underlying pp reference data within their statistical and systematic uncertainties. The sys-tematic uncertainty is dominated by the interpolation method, and is determined by comparing the combined pT- and xT-based reference spectra to the reference spectra obtained solely from the pTor xTdistributions, and also from a

[TeV] s 1 10 ] 3 c -2 [mb GeV 3 /dpσ 3 E d -2 10 -1 10 ) + X -+h + 0.5(h → ) p pp( |<1.0 η | CDF 0.63 TeV CMS 0.9 TeV CDF 1.8 TeV CDF 1.96 TeV CMS 2.76 TeV CMS 7 TeV -interpolation T p = 3 GeV/c T p [TeV] s 1 10 ] 3 c -2 [mb GeV 3 /dpσ 3 E d -7 10 -6 10 -5 10 ) + X -+h + 0.5(h → ) p pp( |<1.0 η | CMS 0.9 TeV CDF 1.96 TeV CMS 2.76 TeV CMS 7 TeV relative placement -interpolation T p -interpolation T x = 15 GeV/c T p 0 100 200 300 400 500 CMS 0.9 TeV CDF 1.96 TeV CMS 2.76 TeV CMS 7 TeV ) + X -+h + 0.5(h → ) p pp( |<1.0 η | = 0.01 T x [TeV] s 1 10 0 1 2 3 4 relative placement -interpolation T x = 0.02 T x ] 3 c -2 [Mb GeV 3 /dpσ 3 E d 4.9 /GeV) s ( 0 [TeV] s 1 0 50 100 150 200 250 300 CMS 0.9 TeV CMS 2.76 TeV CMS 7 TeV ) + X -+h + 0.5(h → ) p pp( |<1.0 η | = 0.03 T x [TeV] s 1 10 0 10 20 30 40 relative placement -interpolation T x = 0.04 T x ] 3_c -2 [kb GeV 3 /dpσ 3 E d 4.9 /GeV) s ( 0

Fig. 2 Examples of interpolations between measured charged-particle

differential cross sections at different√s for pTvalues of 3 and 15 GeV/c (top left and right), xTvalues of 0.01 and 0.02 (bottom left), and xT val-ues of 0.03 and 0.04 (bottom right). These xT values correspond to pT≈ 25, 50, 75, and 100 GeV/c at√s= 5.02 TeV. The second-order polynomial fits, performed in the plane of the log–log invariant

pro-duction vs.√s, are shown by the solid lines. The open squares and circles, and the filled crosses represent interpolated cross section values at 5.02 TeV using different methods: pT-based interpolation, xT-based interpolation, and relative placement, respectively. The error bars on the interpolated points represent the uncertainties in the fit

ence spectrum determined by a “relative placement” method. In the latter, the reference spectrum is obtained by computing where the 5.02 TeV spectrum is situated with respect to the 2.76 and 7 TeV spectra in pythia, and applying the computed placement factors to the measured 2.76 and 7 TeV spectra. The placement factors are determined by taking the value of the 5.02 TeV pythia spectrum, subtracting the value of the 2.76 TeV spectrum, and dividing by the difference between the 7 and the 2.76 TeV spectra. This process is then reversed by using the computed placement factors from pythia, and replacing the 2.76 and 7 TeV pythia spectra with the mea-sured ones to determine the interpolated 5.02 TeV spectrum. Additionally, the NLO-based center-of-mass energy rescal-ing proposed in Ref. [41] is found to yield results consistent within the uncertainties of the other employed methods. The uncertainty in the pp reference distribution due to the

interpo-lation method is estimated to amount to 10 %, which captures the overall point-to-point variations in all of the interpolation and scaling methods employed. The contribution from the uncertainties in the underlying pp input data corresponds to 6 %. These numbers are added in quadrature, resulting in the 12 % uncertainty quoted for the√s= 5.02 TeV interpolated

pp reference spectrum.

3 Systematic uncertainties

A summary of all the contributions to systematic uncer-tainties in the pT spectra, R∗_pPb, and Yasym are given in Table 1. The asterisk symbol is introduced to denote that an interpolated, rather than measured, pp reference spectrum is used to construct the nuclear modification factor. Aside

Table 1 Systematic uncertainties in the measurement of

charged-particle spectra, R_pPb∗ , and Yasym

Source Uncertainty (%)

Trigger efficiency 1.0

Momentum resolution 1.0

Particle species composition 1–10.0 (0.5–5) Track misreconstruction rate 1.0

Track selection 1.2–4.0

Spectra relative normalization 0.0–1.0

Trigger bias 0.0–4.0

Total (spectra) 2.2–10.9

pp interpolation 12.0

Total (R∗_pPb) 12.2–16.2

TpPb average nuclear thickness 4.8 Total (Yasym 0.3 < |ηcm| < 0.8) 2.0–3.0 Total (Yasym 0.8 < |ηcm| < 1.3) 2.0–5.0 Total (Yasym 1.3 < |ηcm| < 1.8) 2.0–5.0 The ranges quoted refer to the variations of the uncertainties as a func-tion of pT. Values in parentheses denote the negative part of the asym-metric uncertainty where applicable. The total uncertainties of the mea-sured pPb and the interpolated pp spectra, as a function of pT, are shown in the lower panel of Fig.3

from the uncertainty from the spectra relative normalization and average nuclear thickness, all uncertainties are deter-mined by taking the approximate maximum deviation from the central value found for the given source. For the parti-cle species uncertainty, an asymmetric uncertainty band is quoted because the maximum deviation above the central value of the measurement is much larger than the maximum deviation below. For the purpose of determining the signif-icance of observed features in the measurement, the uncer-tainties are conservatively treated as following a Gaussian distribution with a root mean square given by the value of the uncertainty as determined above.

The degree of correlation among different uncertainties is described next. For the spectra and R_pPb∗ measurements, the uncertainty in the efficiency of the single-track trigger and offline requirements in selecting DS events is largely a nor-malization uncertainty, although it also slightly affects the shape of the spectrum for pT  3 GeV/c. The uncertainty from the contribution of the various particle species to the unidentified spectrum has the most significant effect in the region 3< pT< 14 GeV/c and can impact the shape of the spectrum in a smooth fashion. At high pT, this effect is less prominent because, due to time dilation, unstable particles have a higher probability of traversing the inner tracker before decaying and therefore a higher probability of being recon-structed. Therefore, from this uncertainty the lower bound on the pPb spectra measurement at higher pTis 2.5 % below the central value, which corresponds to no unstable

parti-] 2 /GeV 2 [c T dpη N/d 2 ) d T pπ 1/(2 ev 1/N₁₀-14 -13 10 -12 10 -11 10 -10 10 -9 10 -8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 10 1 10 2 10 3 10 64) × <-1.3 ( CM η -1.8< 16) × <-0.8 ( CM η -1.3< 4) × <-0.3 ( CM η -0.8< |<1.0 CM η | ) 〉 pPb T 〈 × |<1.0 ( CM η pp reference, | <0.8 (/4) CM η 0.3< <1.3 (/16) CM η 0.8< <1.8 (/64) CM η 1.3< = 5.02 TeV NN s CMS pPb Charged particles -1 L = 35 nb

[GeV/c]

p

Fig. 3 Top Measured charged-particle transverse momentum spectra

in pPb collisions at√s_NN= 5.02 TeV for: |ηcm| < 1.0, 0.3 < ±ηcm< 0.8, 0.8 < ±ηcm < 1.3, and 1.3 < ±ηcm < 1.8, and the interpo-lated pp reference spectrum in|ηcm| < 1, normalized to the number of double-sided events. Positive pseudorapidity values correspond to the proton beam direction. The spectra have been scaled by the quoted factors to provide better visibility. Bottom Systematic uncertainties in the measured pPb and interpolated pp spectra, as a function of pT(see text)

cles being produced. Uncertainty in track misreconstruction can also affect the shape of the measured spectrum, as the misreconstructed fraction of high- pTparticles is sensitive to large occupancy in the silicon tracker within the cones of high-energy jets. The uncertainty in tracking selection can also affect the shape of the spectrum by raising or lowering the measured values at high pT, without changing the

low-pT values, as high- pTtracks are more sensitive to possible mismodeling of detector alignment than low- pTtracks. The uncertainty in the relative normalization of spectra is puted from the normalization factors involved in the com-bination of the pTdistributions from different triggers. This uncertainty only applies for selected pT regions, and may raise or lower the spectrum above pT= 14 GeV/c by a con-stant factor of 1 % relative to the lower- pTpart of the spec-trum. The uncertainty from potential biases of the method used to combine triggers can also affect the shape of the spectrum above pT= 14 GeV/c.

For the R_pPb∗ measurement, the uncertainty in the aver-age nuclear thickness function [3] can influence the R_pPb∗ curve by a constant multiplicative factor. The uncertainty from the pp interpolation is strongly correlated among points

close together in pT, while some partial correlation remains throughout the whole pTregion, even for very different pT values.

For the forward-backward asymmetry measurements, most of these uncertainties cancel in part or in full when the ratio of the spectra is taken. The remaining uncertainty in the detector acceptance and tracking efficiency can change the shape of the forward-backward asymmetry, particularly at high pT.

4 Results

The measured charged-particle yields in double-sided pPb collisions at√s_NN = 5.02 TeV are plotted in Fig.3for the |ηcm| < 1.0, 0.3 < ±ηcm < 0.8, 0.8 < ±ηcm < 1.3, and 1.3 < ±ηcm < 1.8 pseudorapidity ranges. Positive (negative) pseudorapidity values correspond to the proton (lead) beam direction. To improve the visibility of the results, the spectra at different pseudorapidities have been scaled up and down by multiple factors of 4 relative to the data for |ηCM| < 1. The relative uncertainties for the pPb and the pp spectra are given in the bottom panel.

The measurement of the charged-particle nuclear mod-ification factor of Eq. (1) requires a rescaling of the pp cross section by the average nuclear thickness function in minimum-bias pPb collisions. This factor amounts to TpPb = (0.0983 ± 0.0044) mb−1for inelastic pPb colli-sions and is obtained from a Glauber MC simulation [3,42], where the Pb nucleus is described using a Woods-Saxon dis-tribution with nuclear radius 6.62 ± 0.13fm and skin depth of 0.546 ± 0.055fm [3,43]. As double-sided events corre-spond to 94–97 % of inelastic collisions based on HIJING and EPOS MC computations [31], the value ofTpPb would be about 5 % higher for double-sided events.

The charged-particle R∗_pPbat mid-rapidity (|ηcm| < 1) is plotted in Fig.4 as a function of pT. The shaded band at unity and pT≈ 0.6 represents the uncertainty in the Glauber calculation ofTpPb. The smaller uncertainty band around the measured values shows the fully correlated uncertainties from the following sources: spectra relative normalization, track selection, and trigger efficiency. The total systematic uncertainties are shown by the larger band around the mea-sured values (Table1). The nuclear modification factor shows a steady rise to unity at pT≈ 2 GeV/c, then remains constant at unity up to approximately 20 GeV/c, and rises again at higher pT, reaching a maximum value around 1.3–1.4 above 40 GeV/c.

The fact that the nuclear modification factor is below unity for pT  2 GeV/c is anticipated since particle duction in this region is dominated by softer scattering pro-cesses, that are not expected to scale with the nuclear thick-ness function. In the intermediate pTrange (2–5 GeV/c), no

[GeV/c]

p

R*

Fully correlated syst. uncert. Total syst. uncert.

> uncertainty pPb <T |<1 CM η Charged particles, | -1 = 5.02 TeV, L = 35 nb NN s CMS pPb -1 > = 0.0983 mb pPb <T

Fig. 4 Measured nuclear modification factor as a function of pTfor charged particles produced in|ηcm| < 1. The shaded band at unity and pT≈ 0.6 represents the uncertainty in the Glauber calculation of TpPb. The smaller uncertainty band around the data points shows the uncertainty from effects (combining spectra, track selection, and trigger efficiency) that are fully correlated in specific pTregions. The total sys-tematic uncertainties, dominated by uncertainty in the pp interpolation, are shown by the larger band (see Table1)

significant deviation from unity is found in the R∗_pPbratio. There are several prior measurements that suggest an inter-play of multiple effects in this pTregion. At lower collision energies, an enhancement (“Cronin effect” [44]) has been observed [15–18] that is larger for baryons than for mesons, and is stronger in the more central collisions. This enhance-ment has been attributed to a combination of initial-state multiple scattering effects, causing momentum broadening, and hadronization through parton recombination (a final-state effect) [45] invoked to accommodate baryon/meson differences. Recent results from pPb collisions at√sNN = 5.02 TeV [31,46–49] and from dAu collisions at √s_NN = 200 GeV [50,51] suggest that collective effects may also play a role in the intermediate- pTregion. Most theoretical mod-els do not predict a Cronin enhancement in this pTrange at LHC energies as the effect of initial-state multiple scattering is compensated by nPDF shadowing [52].

In Fig. 5, the CMS measurement is compared to the result of an NLO pQCD calculation [53] for charged par-ticles produced at mid-rapidity. The calculation uses the CTEQ10 [54] free-proton PDF, the EPS09 nPDF [4], and the fDSS fragmentation functions [55]. The observed rise of the nuclear modification factor up to R∗_pPb≈ 1.3–1.4 at high

pTis stronger than expected theoretically. None of the avail-able theoretical models [52] predict enhancements beyond

RpPb≈ 1.1 at high pT. In particular, although the pTrange corresponds to parton momentum fractions 0.02  x  0.2, which coincides with the region where parton antishadowing

[GeV/c]

p

, R

R*

NLO (nPDF=EPS09, FF=fDSS), y=0 -1

= 5.02 TeV, L = 35 nb

NN

s CMS pPb

Fully correlated syst. uncert. Total syst. uncert.

|<1 CM η CMS | |<0.3 CM η ALICE | -1 > = 0.0983 mb pPb <T

Fig. 5 Charged-particle nuclear modification factors measured by

CMS in|ηcm| < 1 (filled circles), and by ALICE in |ηcm| < 0.3 (open squares), are compared to the NLO pQCD prediction of Ref. [53]. The theoretical uncertainty is based on the EPS09 error sets. For the CMS measurement, the shaded band at unity and pT ≈ 0.6 represents the uncertainty in the Glauber calculation ofTpPb, the smaller uncertainty band around the data points shows the fully correlated uncertainties and the total systematic uncertainty is shown by the larger band (see Table1). For the ALICE measurement, the total systematic uncertain-ties, excluding the normalization uncertainty of 6 %, are shown with open boxes

effects are expected [10], none of the nPDFs obtained from global fits to nuclear data predict enhancements beyond 10 % at the large virtualities (Q2∼ p2_T∼ 500–10,000 GeV/c2) of relevance here.

An estimate of the significance of this observed rise above unity for 40< pT< 120 GeV/c is determined by interpreting all uncertainties as following a multivariate normal distribu-tion where the components are the six pTbins in the kine-matic region of interest. The variance of each component is given by the sum of the statistical and systematic uncer-tainties in quadrature. For the case of the asymmetric parti-cle species uncertainty, the smaller negative value is used as the data are uniformly larger than the expected values of the hypothesis to be tested. Given that the uncertainties of the ref-erence spectrum are derived from applying different interpo-lation procedures and propagating the uncertainties from pre-vious measurements from multiple experiments, it is not pos-sible to unambiguously determine how all systematic uncer-tainties are correlated between measurements in each pTbin. Therefore, a pair of estimates of the possible significance is given. In one case, only the systematic uncertainties from the relative normalization of the spectra, track selection, trigger efficiency, nuclear thickness function, and NLO pQCD calcu-lation are treated as fully correlated, while others are treated as uncorrelated. In the other case, all systematic uncertainties are treated as fully correlated. Both the hypothesis that R_pPb∗ is

unity and the hypothesis that R_pPb∗ is given by the NLO pQCD calculation are tested. For the case in which some uncertain-ties are treated as uncorrelated, a log-likelihood ratio test is performed using an alternative hypothesis that R_pPb∗ is given by either unity or the NLO prediction, scaled by a constant,

pT-independent, factor. The hypothesis that R∗_pPbis unity for 40< pT< 120 GeV/c is rejected with a p value of 0.006%, and the hypothesis that R_pPb∗ is given by the NLO pQCD calculation for 40 < pT < 120 GeV/c is rejected with a p value of 0.2 %. For the case in which all uncertainties are fully correlated, the log-likelihood ratio test cannot be used, as the covariance matrix becomes nearly singular and the maximum likelihood estimation fails. Instead, a two-tailed univariate test is performed using the single measurement for 61< pT< 74 GeV/c. From this test, the hypothesis that

R∗_pPbis unity for 61< pT < 74 GeV/c is rejected with a p value of 0.4 %, and the hypothesis that R∗_pPbis given by the NLO pQCD calculation for 61< pT< 74 GeV/c is rejected with a p value of 2 %.

Figure 5 also shows the measurement from the ALICE experiment [22], which is performed in a narrower pseudo-rapidity range than the CMS one, and uses a different method (NLO scaling) to obtain the pp reference spectrum based on ALICE pp data measured at√s = 7 TeV. The difference

in the CMS and ALICE R_pPb∗ results stems primarily from differences in the charged-hadron spectra measured in pp collisions at√s= 7 TeV [38,56].

Figure6 shows the forward-backward yield asymmetry,

Yasym(Eq. 2), as a function of pT for 0.3 < |ηcm| < 0.8, 0.8 < |ηcm| < 1.3, and 1.3 < |ηcm| < 1.8. In all three

η ranges, the value of Yasymrises from pT ≈ 0.4 to about 3 GeV/c, then falls to unity at a pTof 10 GeV/c, and remains constant at unity up to the highest pTvalues. At the lowest pT value, Yasymis consistent with unity for 0.3 < |ηcm| < 0.8, but is above unity in the larger pseudorapidity regions. For

pT< 10 GeV/c, the Yasymis larger than unity as has been pre-dicted by models including nuclear shadowing [52]. A the-oretical NLO pQCD computation of Yasymat high pT[53], using CTEQ6 [57] free-proton PDFs, EPS09 nPDFs [4], and Kretzer parton-to-hadron fragmentation functions [58], is also shown in Fig. 6. The theoretical predictions are con-sistent with these data.

To determine if the R∗_pPb and Yasym results can be con-sistently interpreted in terms of nPDF modifications, an MC study using the pythia (Z2 tune) event generator was per-formed to correlate each high- pT hadron to the fractional momentum, x, of the initial-state parton from the Pb nucleus that participated in the hard-scattering process producing the final hadron. In all pseudorapidity intervals studied here, most of the hadrons with pT  20 GeV/c, i.e., in the range where the R∗_pPb exceeds unity in Fig.4, come from the x region that is associated with antishadowing in the nPDF dis-tributions. Although the mean of the x distribution increases

asym

Y

NLO (nPDF=EPS09, FF=KRE)

asym

Y

[GeV/c]

p

Y

Total systematic uncertainty

Fig. 6 Charged-particle forward-backward yield asymmetry as a

func-tion of pTfor 0.3 < |ηcm| < 0.8 (top), 0.8 < |ηcm| < 1.3 (middle), and 1.3 < |ηcm| < 1.8 (bottom). The asymmetry is computed as the charged-particle yields in the direction of the Pb beam divided by those of the proton beam. The solid curves are NLO pQCD theoretical calcu-lations including nPDFs modifications [53]. The theoretical uncertainty is based on the EPS09 error sets

withηcm, for hadrons with pT above 20 GeV/c it remains in the range 0.02  x  0.2. Thus, similar antishadowing effects are expected in the positive and negativeηcmregions resulting in a Yasymclose to unity. At low pT, corresponding to x 0.02, a larger hadron yield is observed in the direction of the Pb beam. This is qualitatively consistent with expec-tations of gluon shadowing [52].

An enhancement in R_pPb∗ at high pT can possibly arise if the quark-jet fraction is larger in pPb than in pp colli-sions. Since the charged-particle products of quark fragmen-tation more often have higher relative pT than those pro-duced by gluon fragmentation, that could lead to an enhance-ment in the charged-particle production at high pT beyond NLO expectations, without a corresponding increase in the jet RpPb[25,26]. We note that the gluon-to-hadron fragmen-tation functions are not well constrained in pp collisions at LHC energies [27], although such uncertainties should largely cancel in ratios of cross sections.

5 Summary

Charged-particle spectra have been measured in pPb col-lisions at√sNN = 5.02 TeV in the transverse momentum

range of 0.4 < pT < 120 GeV/c for pseudorapidities up to |ηcm| = 1.8. The forward-backward yield asymmetry has been measured as a function of pTfor three bins inηcm. At

pT< 10 GeV/c, the charged-particle production is enhanced in the direction of the Pb beam, in qualitative agreement with nuclear shadowing expectations. The nuclear modification factor at mid-rapidity, relative to a reference spectrum inter-polated from pp measurements at lower and higher collision energies, rises above unity at high pTreaching an R_pPb∗ value of 1.3–1.4 at pT  40 GeV/c. The observed enhancement is larger than expected from NLO pQCD predictions that include antishadowing effects in the nuclear PDFs in this kinematic range. Future direct measurement of the spectra of jets and charged particles in pp collisions at a center-of-mass energy of 5.02 TeV is necessary to better constrain the fragmentation functions and also to reduce the dominant sys-tematic uncertainties in the charged-particle nuclear modifi-cation factor.

Acknowledgments We congratulate our colleagues in the CERN accelerator departments for the excellent performance of the LHC and thank the technical and administrative staffs at CERN and at other CMS institutes for their contributions to the success of the CMS effort. In addition, we gratefully acknowledge the computing centres and per-sonnel of the Worldwide LHC Computing Grid for delivering so effec-tively the computing infrastructure essential to our analyses. Finally, we acknowledge the enduring support for the construction and oper-ation of the LHC and the CMS detector provided by the following funding agencies: the Austrian Federal Ministry of Science, Research and Economy and the Austrian Science Fund; the Belgian Fonds de la Recherche Scientifique, and Fonds voor Wetenschappelijk Onder-zoek; the Brazilian Funding Agencies (CNPq, CAPES, FAPERJ, and FAPESP); the Bulgarian Ministry of Education and Science; CERN; the Chinese Academy of Sciences, Ministry of Science and Technol-ogy, and National Natural Science Foundation of China; the Colom-bian Funding Agency (COLCIENCIAS); the Croatian Ministry of Sci-ence, Education and Sport, and the Croatian Science Foundation; the Research Promotion Foundation, Cyprus; the Ministry of Education and Research, Estonian Research Council via 4 and IUT23-6 and European Regional Development Fund, Estonia; the Academy of Finland, Finnish Ministry of Education and Culture, and Helsinki Institute of Physics; the Institut National de Physique Nucléaire et de Physique des Particules/CNRS, and Commissariat à l’Énergie Atom-ique et aux Énergies Alternatives/CEA, France; the Bundesministerium für Bildung und Forschung, Deutsche Forschungsgemeinschaft, and Helmholtz–Gemeinschaft Deutscher Forschungszentren, Germany; the General Secretariat for Research and Technology, Greece; the National Scientific Research Foundation, and National Innovation Office, Hun-gary; the Department of Atomic Energy and the Department of Science and Technology, India; the Institute for Studies in Theoretical Physics and Mathematics, Iran; the Science Foundation, Ireland; the Istituto Nazionale di Fisica Nucleare, Italy; the Ministry of Science, ICT and Future Planning, and National Research Foundation (NRF), Republic of Korea; the Lithuanian Academy of Sciences; the Ministry of Educa-tion, and University of Malaya (Malaysia); the Mexican Funding Agen-cies (CINVESTAV, CONACYT, SEP, and UASLP-FAI); the Ministry of Business, Innovation and Employment, New Zealand; the Pakistan Atomic Energy Commission; the Ministry of Science and Higher Edu-cation and the National Science Centre, Poland; the Fundação para a Ciência e a Tecnologia, Portugal; JINR, Dubna; the Ministry of Edu-cation and Science of the Russian Federation, the Federal Agency of

Atomic Energy of the Russian Federation, Russian Academy of Sci-ences, and the Russian Foundation for Basic Research; the Ministry of Education, Science and Technological Development of Serbia; the Secretaría de Estado de Investigación, Desarrollo e Innovación and Pro-grama Consolider-Ingenio 2010, Spain; the Swiss Funding Agencies (ETH Board, ETH Zurich, PSI, SNF, UniZH, Canton Zurich, and SER); the Ministry of Science and Technology, Taipei; the Thailand Center of Excellence in Physics, the Institute for the Promotion of Teaching Science and Technology of Thailand, Special Task Force for Activat-ing Research and the National Science and Technology Development Agency of Thailand; the Scientific and Technical Research Council of Turkey, and Turkish Atomic Energy Authority; the National Academy of Sciences of Ukraine, and State Fund for Fundamental Researches, Ukraine; the Science and Technology Facilities Council, UK; the US Department of Energy, and the US National Science Foundation. Indi-viduals have received support from the Marie-Curie programme and the European Research Council and EPLANET (European Union); the Lev-entis Foundation; the A. P. Sloan Foundation; the Alexander von Hum-boldt Foundation; the Belgian Federal Science Policy Office; the Fonds pour la Formation à la Recherche dans l’Industrie et dans l’Agriculture (FRIA-Belgium); the Agentschap voor Innovatie door Wetenschap en Technologie (IWT-Belgium); the Ministry of Education, Youth and Sports (MEYS) of the Czech Republic; the Council of Science and Industrial Research, India; the HOMING PLUS programme of Foun-dation for Polish Science, cofinanced from European Union, Regional Development Fund; the Compagnia di San Paolo (Torino); the Con-sorzio per la Fisica (Trieste); MIUR project 20108T4XTM (Italy); the Thalis and Aristeia programmes cofinanced by EU-ESF and the Greek NSRF; and the National Priorities Research Program by Qatar National Research Fund.

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CMS Collaboration

Yerevan Physics Institute, Yerevan, Armenia

V. Khachatryan, A. M. Sirunyan, A. Tumasyan

Institut für Hochenergiephysik der OeAW, Wien, Austria

W. Adam, T. Bergauer, M. Dragicevic, J. Erö, M. Friedl, R. Frühwirth1_{, V. M. Ghete, C. Hartl, N. Hörmann, J. Hrubec,} M. Jeitler1, W. Kiesenhofer, V. Knünz, M. Krammer1, I. Krätschmer, D. Liko, I. Mikulec, D. Rabady2, B. Rahbaran, H. Rohringer, R. Schöfbeck, J. Strauss, W. Treberer-Treberspurg, W. Waltenberger, C.-E. Wulz1

National Centre for Particle and High Energy Physics, Minsk, Belarus

V. Mossolov, N. Shumeiko, J. Suarez Gonzalez

Universiteit Antwerpen, Antwerpen, Belgium

S. Alderweireldt, S. Bansal, T. Cornelis, E. A. De Wolf, X. Janssen, A. Knutsson, J. Lauwers, S. Luyckx, S. Ochesanu, R. Rougny, M. Van De Klundert, H. Van Haevermaet, P. Van Mechelen, N. Van Remortel, A. Van Spilbeeck

Vrije Universiteit Brussel, Brussels, Belgium

F. Blekman, S. Blyweert, J. D’Hondt, N. Daci, N. Heracleous, J. Keaveney, S. Lowette, M. Maes, A. Olbrechts, Q. Python, D. Strom, S. Tavernier, W. Van Doninck, P. Van Mulders, G. P. Van Onsem, I. Villella

Université Libre de Bruxelles, Bruxelles, Belgium

C. Caillol, B. Clerbaux, G. De Lentdecker, D. Dobur, L. Favart, A. P. R. Gay, A. Grebenyuk, A. Léonard, A. Mohammadi, L. Perniè2, A. Randle-conde, T. Reis, T. Seva, L. Thomas, C. Vander Velde, P. Vanlaer, J. Wang, F. Zenoni

Ghent University, Ghent, Belgium

V. Adler, K. Beernaert, L. Benucci, A. Cimmino, S. Costantini, S. Crucy, S. Dildick, A. Fagot, G. Garcia, J. Mccartin, A. A. Ocampo Rios, D. Ryckbosch, S. Salva Diblen, M. Sigamani, N. Strobbe, F. Thyssen, M. Tytgat, E. Yazgan, N. Zaganidis

Université Catholique de Louvain, Louvain-la-Neuve, Belgium

S. Basegmez, C. Beluffi3, G. Bruno, R. Castello, A. Caudron, L. Ceard, G. G. Da Silveira, C. Delaere, T. du Pree, D. Favart, L. Forthomme, A. Giammanco4, J. Hollar, A. Jafari, P. Jez, M. Komm, V. Lemaitre, C. Nuttens, D. Pagano, L. Perrini, A. Pin, K. Piotrzkowski, A. Popov5, L. Quertenmont, M. Selvaggi, M. Vidal Marono, J. M. Vizan Garcia

Université de Mons, Mons, Belgium

N. Beliy, T. Caebergs, E. Daubie, G. H. Hammad

Centro Brasileiro de Pesquisas Fisicas, Rio de Janeiro, Brazil

W. L. Aldá Júnior, G. A. Alves, L. Brito, M. Correa Martins Junior, T. Dos Reis Martins, C. Mora Herrera, M. E. Pol

Universidade do Estado do Rio de Janeiro, Rio de Janeiro, Brazil

W. Carvalho, J. Chinellato6, A. Custódio, E. M. Da Costa, D. De Jesus Damiao, C. De Oliveira Martins,

S. Fonseca De Souza, H. Malbouisson, D. Matos Figueiredo, L. Mundim, H. Nogima, W. L. Prado Da Silva, J. Santaolalla, A. Santoro, A. Sznajder, E. J. Tonelli Manganote6, A. Vilela Pereira

Universidade Estadual Paulistaa, Universidade Federal do ABCb, São Paulo, Brazil

C. A. Bernardesb, S. Dograa, T. R. Fernandez Perez Tomeia, E. M. Gregoresb, P. G. Mercadanteb, S. F. Novaesa, Sandra S. Padulaa

Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria

A. Aleksandrov, V. Genchev2, R. Hadjiiska, P. Iaydjiev, A. Marinov, S. Piperov, M. Rodozov, G. Sultanov, M. Vutova

University of Sofia, Sofia, Bulgaria

A. Dimitrov, I. Glushkov, L. Litov, B. Pavlov, P. Petkov

Institute of High Energy Physics, Beijing, China